Pi Zero Tesla coil- GDT , OCD & Phase Transformers

From the main board there are two ferrite transformers. All Three are wound on FT140-43 FAIR-RITE ferrite torrids

The Gate Drive Transformer

This is a high frequency transformer that takes the IGBT drive from the board and delivers it to the IGBT gates. There are 5 winding on this transformer. One which is the primary and is connected to the drivers on the main board and 4 secondary windings that go to drive the IGBT’s. The Tranformer has 16 Turns on each winding. The transformer is wound to reduce the parasitic inductance between windings and maximise the coupling. This is achieved by taking 5 lengths of 0.33MM enameled wire, securing one end and placing the other in a drill and winding them together. This bundle is then wound around the core evenly spacing out the turns over the area of the core. One wound the individual windings are identified with a multi-meter and split out. It is vital that the phasing of the winding is maintained, I do this by tying a knot in all of the wires from one end of the bundle so when they are split out the phase is obvious. These wires are connected to the bridge.

The Phase Current Transformer

This is wound as a current transformer, there are roughly 80 turns of 0.33mm enameled wire on this core and the high current drive from the bridge passes through the middle making a single turn. This transformer is designed to give a phase signal related to the current in the primary of the Tesla Coil. It is wound by taking two lengths of enameled wire and winding the two together for roughly 40 turns, they can be broken out and joined together (red sleeve above) to make a single winding of 80 turns. It is done this way, as winding a long single length of wire onto a torroid becomes really difficult.

The Over Current Detect Transformer

This is again wound as a current transformer, there are roughly 100 turns of 0.33mm enameled wire on this core and the high current drive from the bridge passes through the middle making a single turn. This transformer is designed to give a current signal proportional to current in the primary of the Tesla Coil. It is wound by taking three lengths of enameled wire and winding the two together for roughly 33 turns, they can be broken out and joined together (red sleeve above) to make a single winding of 100 turns. Again,it is done this way to avoid winding a long single length of wire onto the torroid. The output of this coil is fed to the main board which has a 5Ohm Burden resistor. The combination of these two makes for a voltage of 5V across the burden for every 100A of primary current.